Advances in photolithographic techniques have been determinant in the success of the chip industry by allowing a continuous scaling down in the physical dimensions of the structures of integrated components, and consequently, in the cost of semiconductor devices.
These results have been achieved by using radiation of smaller and smaller wavelengths (λ), and increasingly large numerical apertures (NA) of the optical system by implementing techniques based on phase contrast lithographic masks (PFM) and off-axis or annular illumination and use of high performance resist. The resolution of the optical system is proportional to the ratio (λ/NA).
Basically, photolithographic processing implies the following steps:
1. realization of either a binary (chromium and quartz) or a phase-contrast lithographic mask;
2. deposition of a layer of resist over the semiconductor wafer (commonly a slice of monocrystalline silicon);
3. exposing of the resist through the photolithographic mask for transferring the image thereof (usually focused by a projection system of lenses) on the photoresist;
4. development of the exposed photoresist;
5. performance of a plasma etch and/or an ion implanting step through the openings of the developed resist mask;
6. removal of the resist; and
7. dimensional checking of the defined features.
FIGS. 1a, 1b, 1c, 1d, 1e and 1f depict the processing steps 2, 3, 4, 5 and 6. The ability of reducing the physical dimensions of the lithographically defined features on the wafer is limited by the technological ability of lithographically defining smaller and smaller details.
To this end, electron lithography techniques have been developed. In case of the PREVAIL technique from IBM, the radiation source produces an electron beam which illuminates a stencil mask, and the electrons passing through the apertures of the mask are focused by a projection system of lenses (operating at a 4× reduction) onto the resist layer. The advantage of this system rests on the relatively small wavelength of the electron beam, and therefore, on an inherent possibility of allowing enhanced levels of definition, and therefore, compactness of the integrated structures.
A different approach that is being followed is referred to as direct writing of the resist by an electron brush without the use of any mask. According to this approach, a focused electron beam produces the image in the resist. Of course, this means that the various features of the semiconductor device are defined one by one by the electron brush. Although resolution is very high, the method is characterized by an extreme slowness (4-8 wafers/hour) and its productivity will probably remain relatively low. Notwithstanding this intrinsic drawback, many manufacturers are favoring this technology for supporting the development of future generation devices.
Besides the cost of apparatus and infrastructure, advances in the technological limits of lithographic processings implies an ever increasing complexity of the processing operations that become more and more time consuming.